Absorbent foam

ABSTRACT

Disclosed is an absorbent foam that exhibits desirable softness and flexibility properties yet is highly absorbent. In one embodiment, the absorbent foam comprises a water-swellable, water-insoluble polymer wherein the absorbent foam exhibits a Free Swell value of at least about 10 grams of liquid per gram of absorbent foam and a Softness value that is less than about 30 grams of force per gram per square meter of absorbent foam. In a second embodiment, the absorbent foam has an average cell size of the cells in the absorbent foam between about 10 microns to about 100 microns and an average wall thickness of the cells in the absorbent foam between about 0.1 micron to about 30 microns. Such an absorbent foam may be used in a disposable absorbent product intended for the absorption of fluids such as body fluids.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an absorbent foam that exhibitsdesirable softness and flexibility properties yet is highly absorbent.Such an absorbent foam may be used in a disposable absorbent productintended for the absorption of fluids such as body fluids.

2. Description of the Related Art

Disposable absorbent products currently find widespread use in manyapplications. For example, in the infant and child care areas, diapersand training pants have generally replaced reusable cloth absorbentarticles. Other typical disposable absorbent products include femininecare products such as sanitary napkins or tampons, adult incontinenceproducts, and health care products such as surgical drapes or wounddressings. A typical disposable absorbent product generally comprises acomposite structure including a topsheet, a backsheet, and an absorbentstructure between the topsheet and backsheet. These products usuallyinclude some type of fastening system for fitting the product onto thewearer.

The use of water-swellable, generally water-insoluble absorbentmaterials, commonly known as superabsorbents, in disposable absorbentpersonal care products is known. Such absorbent materials are generallyemployed in absorbent products in order to increase the absorbentcapacity of such products while reducing their overall bulk. Suchabsorbent materials are generally present in absorbent products in theform of small particles in a fibrous matrix, such as a matrix of woodpulp fluff. A matrix of wood pulp fluff generally has an absorbentcapacity of about 6 grams of liquid per gram of fluff. Thesuperabsorbent materials generally have an absorbent capacity of atleast about 10, preferably of about 20, and often of up to 100 timestheir weight in water. Clearly, incorporation of such absorbentmaterials in disposable absorbent products can reduce the overall bulkwhile increasing the absorbent capacity of such products.

As an alternative to using a fibrous matrix containing superabsorbentmaterials, absorbent foam composites are also known. One form of anabsorbent foam composite is wherein a foam material, such aspolyurethane, is prepared to include a particulate superabsorbentmaterial within the structure of the polyurethane foam. Alternatively, aparticulate superabsorbent material is located between at least twolayers of a polyurethane foam material to form a layered compositestructure. While such foam structures may be useful absorbent materialsin specific applications, they have not been shown to be optimal for usein disposable absorbent products because their absorptive propertiestend to be limited. In particular, the foam material in such structures,such as polyurethane, generally does not have a sufficient absorptiveability to retain liquids. Therefore, although the particulatesuperabsorbent material in the foam structure may be able to retain aliquid, the overall capacity of the foam structure to absorb and retaina liquid is limited. Furthermore, the overall absorptive properties ofthe foam structure tend to be limited due to the relatively low surfacearea to mass ratio of the particulate superabsorbent material portionrelative to the foam portion of the structure.

Absorbent foams are also known that are prepared comprising essentiallyall superabsorbent material. Typically, a blowing agent is used to forma foamed, water-swellable, polymeric liquid absorbent material. However,certain absorbent foams prepared using specific blowing agents have beenfound to have limited use for liquid absorption or liquid distribution.This is typically due to physical characteristics of the foam structure,which may include discontinuous channels, a too large average cell size,unacceptably wide cell size distribution, and/or capillary diametersthat vary widely and randomly, that tend to result in undesirableabsorptive rates and capacities and undesirable liquid distributionproperties. In addition, known absorbent foams that are preparedcomprising essentially all superabsorbent material have typically beenfound to have undesirable non-absorptive physical characteristics suchas a lack of softness or being too brittle. Furthermore, many of theknown foams are hydrophobic in nature and need treatment with a wettingagent or other suitable treatment steps to obtain a hydrophilic nature.Such undesirable non-absorptive physical characteristics of an absorbentfoam tends to limit the usefulness of the absorbent foam in disposableabsorbent products since such disposable absorbent products generallyneed to be sufficiently flexible to withstand the rigors of use by aconsumer and also be sufficiently soft to be acceptably comfortableduring use.

Thus, there is a continuing need for improvement of absorbent foams. Inparticular, there is a need for an absorbent foam which exhibits arelatively high absorptive liquid capacity yet which exhibits desirablesoftness and flexibility properties.

It is therefore an object of the present invention to provide anabsorbent foam which exhibits a relatively high absorptive liquidcapacity yet which exhibits desirable physical characteristics such assoftness and flexibility properties.

It is also an object of the present invention to provide a disposableabsorbent product which includes an absorbent foam that exhibits arelatively high absorptive liquid capacity yet which exhibits desirablephysical characteristics such as softness and flexibility properties.

SUMMARY OF THE INVENTION

The present invention concerns an absorbent foam which exhibits arelatively high absorptive liquid capacity yet which exhibits desirablephysical characteristics such as softness and flexibility properties.

One aspect of the present invention concerns an absorbent foam thatcomprises a water-swellable, water-insoluble polymer wherein theabsorbent foam exhibits a Free Swell value of at least about 10 grams ofliquid per gram of absorbent foam and a Softness value that is less thanabout 30 grams of force per gram per square meter of absorbent foam.

In another aspect, the present invention concerns a absorbent foam thatcomprises a water-swellable, water-insoluble polymer wherein theabsorbent foam has an average cell size of the cells in the absorbentfoam between about 10 microns to about 100 microns and an average wallthickness of the cells in the absorbent foam between about 0.1 micron toabout 30 microns.

In another aspect, the present invention concerns a disposable absorbentproduct comprising the absorbent foam disclosed herein.

One embodiment of such a disposable absorbent product comprises aliquid-permeable topsheet, a backsheet attached to the liquid-permeabletopsheet, and an absorbent foam of the present invention located betweenthe liquid-permeable topsheet and the backsheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the equipment employed in determining theFree Swell and Absorbency Under Load values of an absorbent foam ormaterial.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an absorbent foam which exhibits arelatively high absorptive liquid capacity yet which exhibits desirablesoftness and flexibility properties. The absorbent foam comprises awater-swellable, water-insoluble polymer. As used in the presentinvention, the water-swellable, water-insoluble polymer to a largeextent needs to provide the absorbent foam with its liquid-absorbingcapacity. As such, the water-swellable, water-insoluble polymer needs tobe effective to provide a desired amount of liquid-absorbing capacity tothe absorbent foam.

As used herein, the term "foam" is generally intended to represent aporous polymeric matrix, which is an aggregate of hollow cells, theboundaries or walls of which cells comprise solid polymeric material.The cells may be interconnected to form channels or capillaries withinthe foam structure wherein such channels or capillaries facilitateliquid distribution within the foam.

As used herein, the term "water-swellable, water-insoluble" is meant torefer to a material that, when exposed to an excess of water, swells toits equilibrium volume but does not dissolve into the water. As such, awater-swellable, water-insoluble material generally retains its originalidentity or physical structure, but in a highly expanded state, duringthe absorption of the water and, thus, must have sufficient physicalintegrity to resist flow and fusion with neighboring materials.

As used herein, a material will be considered to be "water soluble" whenit substantially dissolves in excess water to form a solution, therebylosing its initial form and becoming essentially molecularly dispersedthroughout the water solution. As a general rule, a water-solublematerial will be free from a substantial degree of crosslinking, ascrosslinking tends to render a material water insoluble.

Polymers which are suitable for use in the present invention aregenerally any polymer which is initially soluble in a solvent such thatthe soluble polymer may be formed into a solution by mixing with aliquid solvent, such as water, and then whereby the polymer is treatedto cause the polymer to become water-swellable and water-insoluble sothat an absorbent foam comprising such water-swellable, water-insolublepolymer exhibits desired absorbency and physical characteristics.

Polymers which are suitable for use in the present invention include awide variety of anionic, cationic, and nonionic materials. Suitablepolymers include polyacrylamides, polyvinyl alcohols, ethylene maleicanhydride copolymer, polyvinylethers, polyacrylic acids,polyvinylpyrrolidones, polyvinylmorpholines, polyamines,polyethyleneimines, polyquaternary ammoniums, natural basedpolysaccharide polymers such as carboxymethyl celluloses, carboxymethylstarchs, hydroxypropyl celluloses, algins, alginates, carrageenans,acrylic grafted starchs, acrylic grafted celluloses, chitin, andchitosan, and synthetic polypeptides such as polyaspartic acid,polyglutamic acid, polyasparagins, polyglutamines, polylysines, andpolyarginines, as well as the salts, copolymers, and mixtures of any ofthe foregoing polymers.

In one embodiment of the present invention, it is desired that thepolymer used be a glassy polymer. As used herein, the term "glassy"polymer is meant to refer to a polymer having a glass transitiontemperature (Tg) above about 23° C. (about room temperature) at arelative humidity of about 30 percent or less. Examples of glassypolymers include, but are not limited to, sodium polyacrylate,polyacrylic acid, sodium carboxymethyl cellulose, and chitosan saltpolymers. Examples of non-glassy polymers include, but are not limitedto, polyethylene oxide, polyvinyl acetate, and polyvinyl ether polymers.

One property of the water-swellable, water-insoluble polymer which isrelevant to its effectiveness in providing a desired amount ofliquid-absorbing capacity to an absorbent foam is its molecular weight.In general, a water-swellable, water-insoluble polymer with a highermolecular weight will exhibit a higher liquid-absorbing capacity ascompared to a water-swellable, water-insoluble polymer with a lowermolecular weight.

The water-swellable, water-insoluble polymer useful in the absorbentfoam of the present invention may generally have a wide range ofmolecular weights. A water-swellable, water-insoluble polymer having arelatively high molecular weight is often beneficial for use in thepresent invention. Nonetheless, a wide range of molecular weights isgenerally suitable for use in the present invention. Water-swellable,water-insoluble polymers suitable for use in the present invention willbeneficially have a weight average molecular weight greater than about10,000, more beneficially greater than about 100,000, even morebeneficially greater than about 200,000, suitably greater than about500,000, more suitably greater than about 1,000,000, and up to about20,000,000. Methods for determining the molecular weight of a polymerare well-known in the art.

It is generally desired that the polymer be present in the absorbentfoam in an amount effective to result in the absorbent foam exhibitingdesired properties. The polymer will be present in the absorbent foam ina weight amount that is between about 50 weight percent to 100 weightpercent, beneficially between about 60 weight percent to about 100weight percent, more beneficially between about 70 weight percent toabout 100 weight percent, suitably between about 80 weight percent toabout 100 weight percent, more suitably between about 90 weight percentto about 100 weight percent, and even more suitably between about 95weight percent to about 100 weight percent, wherein all weight percentsare based on the total weight amount of the polymer, any crosslinkingagents, and any other optional components present in the absorbent foam.In one embodiment of the present invention, it is desired that theabsorbent foam consist essentially of the polymer and, optionally, anycrosslinking agent used to crosslink the polymer. As will be appreciatedby one skilled in the art, such an absorbent foam may also comprise aninsubstantial amount of solvent retained from the preparation processand/or an insubstantial amount of water vapor absorbed from the air. Ingeneral, the presence of any materials in the absorbent foam that arenot the water-swellable, water-insoluble polymer will tend to reduce theoverall liquid absorbency capacity of the absorbent foam.

The water-swellable, water-insoluble polymer useful in the absorbentfoam will generally be crosslinked. The amount of crosslinking shouldgenerally be above a minimum amount sufficient to make the polymerwater-insoluble but also below some maximum amount so as to allow thepolymer to be sufficiently water swellable so that the water-swellable,water-insoluble polymer absorbs a desired amount of liquid absorption.

Crosslinking of the polymer may generally occur either while the polymeris in solution or after the solvent has been removed from a solutionused to prepare the absorbent foam. Such crosslinking of the polymer maygenerally be achieved by either of two different types of crosslinkingagents. Such crosslinking agents will generally be soluble in thesolvent being used, such as water.

One type of crosslinking agent is a latent crosslinking agent. Suitablelatent crosslinking agents are generally either internal latentcrosslinking agents or external latent crosslinking agents. An internallatent crosslinking agent is generally copolymerizable to the monomer ormonomers used to prepare the polymer and, thus, generally comprise atleast one vinyl group and one functional group or functionality that iscapable of reacting with the side groups on the base polymer, such as acarboxyl group (--COO⁻) on a sodium polyacrylate polymer or a carboxylicacid group (--COOH) on a polyacrylic acid polymer. Examples of suitablecopolymerizable crosslinking agents include ethylenically unsaturatedmonomers, such as ethylene glycol vinyl ether and amino propyl vinylether.

An external latent crosslinking agent generally crosslinks the polymeritself after, for example, a polymer has been formed from specificmonomer or monomers used to prepare the polymer and/or a polymer hasbeen mixed with a solvent to form a solution. Latent crosslinking agentsgenerally do not take part in the overall polymerization process but,instead, are reactive to the polymer at a later point in time when aproper crosslinking condition is provided. Suitable crosslinkingconditions include using heat treatment, such as a temperature aboveabout 60° C., exposure to ultraviolet light, exposure to microwaves,steam or high humidity treatment, high pressure treatment, or treatmentwith an organic solvent.

Suitable external latent crosslinking agents are any organic compoundhaving at least two functional groups or functionalities capable ofreacting with the carboxyl, carboxylic acid, amino, or hydroxyl groupsof a polymer. It is desired that such an organic crosslinking agent beselected from the group consisting of diamines, polyamines, diols, andpolyols and mixtures thereof; particularly from the group consisting ofprimary diols, primary polyols, primary diamines and primary polyaminesand mixtures thereof. Of the diols and polyols, those possessing longer,such as 4 or greater, carbon chain lengths are generally beneficial.Specifically, the crosslinking agent may be selected from the groupconsisting of chitosan glutamate, type A gelatin, diethylenetriamine,ethylene glycol, butylene glycol, polyvinyl alcohol, hyaluronic acid,polyethylene imine and their derivatives and mixtures thereof. Othersuitable organic crosslinking agents include monochloroacetic acid,sodium chloroacetate, citric acid, butane tetracarboxylic acid, andamino acids such as aspartic acid, and mixtures thereof. Anothersuitable latent crosslinking agent comprises a metal ion with more thantwo positive charges, such as Al³⁺, Fe³⁺, Ce³⁺, Ce⁴⁺, Ti⁴⁺, Zr⁴⁺, andCr³⁺. Suitable metal ion crosslinking agents include those of thetransition elements which generally have vacant d-orbitals. Suitablemetal ion crosslinking agents include AlCl₃, FeCl₃, Ce₂ (SO₄)₃, Zr(NH₄)₄(CO₃)₄ and Ce(NH₄)₄ (SO₄)₄.2H₂ O, other well known metal ion compoundsand mixtures thereof. Such metal ion crosslinking agents, when used witha particular polymer, are believed to form ionic bonds with thecarboxyl, carboxylic, amino, or hydroxyl groups on the polymer. Metalions with only two positive charges, such as Zn²⁺, Ca²⁺, or Mg²⁺, arealso suitable as crosslinking agents for certain polymers.

When the polymer is a cationic polymer, a suitable crosslinking agent isa polyanionic material such as sodium polyacrylate, carboxymethylcellulose, or polyphosphate.

A second type of crosslinking mechanism that certain polymers are ableto undergo involves a macromolecular rearrangement of the chains of thepolymer during the solidification process of the polymer such that thepolymer forms a higher ordered structure with a high degree ofcrystallinity which is generally water insoluble. Polymers suitable tosuch a crosslinking approach include, but are not limited to, polyvinylalcohol, chitosan, and carboxymethyl cellulose with a relatively lowdegree of carboxymethylation. Additional strong bonding of the polymercould be established between the polymer chains during thesolidification process which could result in a generally water insolublematerial. An example of this behavior is the strong hydrogen bonding inpolyvinyl alcohol forming an insoluble material.

Suitable crosslinking agents for a polymer solution gel process are alsogenerally of two different types: either internal polymerizable orexternal crosslinking agent. The first type of crosslinking agent is apolymerizable but instant crosslinking agent. Suitable polymerizablecrosslinking agents are generally reactive to the monomer or monomersused to prepare the polymer and, thus, generally comprise at least twofunctional groups or functionalities that are capable of reacting withthe monomers. Examples of suitable polymerizable crosslinking agentsinclude ethylenically unsaturated monomers, such as N,N'-methylenebis-acrylamide for free radical polymerization, and polyamines orpolyols for condensation polymerization. The second type of crosslinkingagent is a reactive compound having at least two functional groups orfunctionalities capable of reacting with the carboxyl, carboxylic acid,amino, or hydroxyl groups of a polymer in the solution stage whereinsuch crosslinking is not latent, in that no additional conditions areneeded to initialize the crosslinking reaction. Suitable crosslinkingagents may be selected from the group consisting of aldehydes, such asglutaraldehyde, or glycidyl ethers, such as polyethylene gylcoldiglycidyl ether.

Another approach to form a crosslinked polymer network in either apolymer solution or on a recovered polymer is the use of a high energytreatment such as electron beam radiation or microwave radiation to formfree radicals in the polymer which are then used to generatecrosslinking points. This approach is applicable but not limited toinstances where a crosslinking agent is not used to prepare theabsorbent foam.

If a crosslinking agent is used, it is generally desired that thecrosslinking agent be used in an amount that is beneficially from about0.01 weight percent to about 20 weight percent, more beneficially fromabout 0.05 weight percent to about 10 weight percent, and suitably fromabout 0.1 weight percent to about 5 weight percent, based on the totalweight of the polymer and the crosslinking agent present in an absorbentfoam.

In general, a crosslinking catalyst will not be needed, but may bebeneficial, to assist in the crosslinking of the polymer in order toprepare the absorbent foam of the present invention. For example, ifcitric acid is used as the crosslinking agent, sodium hypophosphite isbeneficially used as a crosslinking catalyst. If a crosslinking catalystis used, it is generally desired that the crosslinking catalyst be usedin an amount of from about 0.01 to about 3 weight percent, suitably fromabout 0.1 to about 1 weight percent, based on the total weight of thepolymer used.

While the principal components of the absorbent foam of the presentinvention have been described in the foregoing, such an absorbent foamis not limited thereto and can include other components not adverselyeffecting the desired properties of the absorbent foam. Exemplarymaterials which could be used as additional components would include,without limitation, pigments, antioxidants, stabilizers, plasticizers,nucleating agents, surfactants, waxes, flow promoters, solid solvents,particulates, and materials added to enhance processability of theabsorbent foam. If such additional components are included in anabsorbent foam, it is generally desired that such additional componentsbe used in an amount that is beneficially less than about 10 weightpercent, more beneficially less than about 5 weight percent, andsuitably less than about 1 weight percent, wherein all weight percentsare based on the total weight amount of the amount of the polymer, anycrosslinking agents, and any other optional components present in theabsorbent foam.

The absorbent foam of the present invention suitably has the ability toabsorb a liquid, herein referred to as the Free Swell (FS) value. Themethod by which the Free Swell value is determined is set forth below inconnection with the examples. The Free Swell values determined as setforth below and reported herein refer to the amount in grams of anaqueous solution, containing 0.9 weight percent sodium chloride, a gramof a material can absorb in about 1 hour under a negligible load ofabout 0.01 pound per square inch (psi). As a general rule, it is desiredthat the absorbent foam of the present invention has a Free Swell value,for a load of about 0.01 psi, of at least about 10, beneficially of atleast about 15, more beneficially of at least about 20, suitably of atleast about 25, more suitably of at least about 30, and up to about 200grams per gram of absorbent foam.

The absorbent foam of the present invention also suitably has theability to absorb a liquid while the absorbent composition is under anexternal pressure or load, herein referred to as the Absorbency UnderLoad (AUL) value. The ability of a material to absorb a liquid while theabsorbent composition is under an external pressure or load has beenfound to often be an important characteristic of an absorbent materialused in a disposable absorbent product since, while being worn and/orused by a consumer, the disposable absorbent product is often subjectedto an external pressure or load that may negatively impact on theability of the absorbent material being used to effectively absorb anyliquid insulting the disposable absorbent product. The method by whichthe Absorbency Under Load is determined is set forth below in connectionwith the examples. The Absorbency Under Load values determined as setforth below and reported herein refer to the amount in grams of anaqueous solution, containing 0.9 weight percent sodium chloride, a gramof a material can absorb in about 1 hour under a load of about 0.3 poundper square inch (psi). As a general rule, it is desired that theabsorbent foam of the present invention has an Absorbency Under Loadvalue, for a load of about 0.3 psi, of at least about 10, beneficiallyof at least about 15, more beneficially of at least about 20, suitablyof at least about 25, more suitably of at least about 30, and up toabout 100 grams per gram of absorbent foam.

It has been discovered that the conditions under which an absorbent foamis stored may potentially have an impact on the absorbent properties ofthe absorbent foam as it ages. Even relatively mild conditions, such asambient conditions, such as about 24° C. and at least about 30 percentrelative humidity, suitably between about 30 to about 60 percentrelative humidity, may result in a degradation of the absorbentproperties of an absorbent foam as it ages. Typically, storageconditions, such as relatively higher temperatures and/or relativelyhigher relative humidities, as compared to ambient conditions, mayresult in quicker and/or more severe degradation of the absorbentproperties of an absorbent foam as it ages.

In one embodiment of the present invention, the absorbent foam of thepresent invention will tend to retain its initial Free Swell and AULvalues after aging. Specifically, an absorbent foam of the presentinvention may retain greater than about 50 percent, and suitably greaterthan about 70 percent, of its initial Free Swell or AUL values afteraging for about 60 days. Typically, the aging conditions are at ambientconditions, such as at about 24° C. and at least about 30 percentrelative humidity. For example, if an absorbent foam of the presentinvention has an initial AUL value of about 20, that absorbent foam mayhave an AUL value of at least about 10, and suitably of about 14, afteraging for about 60 days at about 24° C. and at least about 30 percentrelative humidity. Otherwise similar absorbent foams may tend to notretain their initial Free Swell or AUL values after aging under similarconditions.

Suitably, the absorbent foam of the present invention retains greaterthan about 50 percent, and more suitably greater than about 70 percent,of their initial Free Swell and AUL values after aging for about 60 daysat about 24° C. and about 100 percent relative humidity.

As used herein, the term "initial Free Swell" or "initial AbsorbencyUnder Load" is meant to refer to that Free Swell or AUL value exhibitedby an absorbent foam as measured within about 1 day after preparation ofthe absorbent foam when the absorbent foam is stored at ambientconditions, such as at about 24° C. and between about 30 to about 60percent relative humidity.

It is also desirable that the absorbent foam of the present inventionexhibit addition liquid handling properties such as suitable liquidvertical wicking or liquid intake rate values.

The absorbent foam of the present invention also suitably exhibitsdesired softness characteristics, herein quantified by the use of aSoftness value. It is generally desired to have an absorbent foam thatis soft and flexible so that a disposable absorbent product comprisingthe absorbent foam will provide a good fit to a wearer or user of thedisposable absorbent product so as to prevent premature liquid leakage,a certain degree of comfort, and a reduced packaging volume because asoft material generally provides a maximum compressibility and foldingcapacity. The method by which the Softness value is determined is setforth below in connection with the examples. The Softness valuesdetermined as set forth below and reported herein refer to the forcevalue that relates to the stiffness of a material. Using the test methoddescribed herein, the Softness value of a material gives an average ofthe stiffness of the material in all directions, and is a measurement offorce exerted on the material at a rate of 50 centimeters per minute, ina circular bending test. In general, the higher the force value neededto bend a material, the more stiff the material is. As a general rule,it is desired that the absorbent foam of the present invention exhibitsa Softness value that is beneficially less than about 30, morebeneficially less than about 25, even more beneficially less than about20, suitably less than about 15, more suitably less than about 10, andeven more suitably less than about 5 grams of force per gram per squaremeter of absorbent foam.

A typical foam will comprise open spaces or cells within the structureof the foam. In the development of the present invention, it has beendetermined that the size of the cells of an absorbent foam generallyaffects certain liquid transportation properties, such as verticalliquid wicking values, but that the size of the cells of an absorbentfoam generally has a minimal affect on the overall softness orflexibility of the absorbent foam. Such has been found to beparticularly true when the polymer being used to prepare the absorbentfoam is a glassy polymer. Instead, the softness or flexibility of anabsorbent foam has been found to be generally dependent on the thicknessof the cell walls. In general, the thinner the wall thickness of thecells of an absorbent foam, the softer and/or more flexible theabsorbent foam will be. In order to achieve the desired absorbency andphysical characteristics of the absorbent foam of the present invention,it has been found that both the average cell size and the averagethickness of the cell walls of an absorbent foam needs to be carefullycontrolled and, preferably, optimized.

In one embodiment of the present invention, it is generally desired thatthe average cell size of the cells in an absorbent foam beneficially bebetween about 10 microns to about 100 microns and suitably between about10 microns to about 50 microns. Such a range of the average cell size ofthe cells in an absorbent foam has been found to generally result in aneffective channel system for distributing liquid within the structure ofthe absorbent foam. The method by which the average cell size of thepores in an absorbent foam is determined is set forth below inconnection with the examples.

In one embodiment of the present invention, it is generally desired thatthe average wall thickness of the cells in an absorbent foambeneficially be between about 0.1 micron to about 30 microns andsuitably between about 0.5 micron to about 10 microns. Such a range ofthe wall thickness of the cells in an absorbent foam has been found togenerally result in achieving desired physical properties, such assoftness and/or flexibility, of the absorbent foam. The method by whichthe average wall thickness of the pores in an absorbent foam isdetermined is set forth below in connection with the examples.

As used herein, the term "hydrophobic" refers to a material having acontact angle of water in air of at least 90 degrees. In contrast, asused herein, the term "hydrophilic" refers to a material having acontact angle of water in air of less than 90 degrees. For the purposesof this application, contact angle measurements are determined as setforth in Robert J. Good and Robert J. Stromberg, Ed., in "Surface andColloid Science--Experimental Methods", Vol. II, (Plenum Press, 1979).The absorbent foams of the present invention are generally hydrophilicas prepared and therefore generally do not require any subsequenttreatment to make them hydrophilic. This is in contrast to manyabsorbent foams known in the art in which the polymeric material of thefoam is not inherently hydrophilic but is rendered hydrophilic by asuitable treatment, such as by the addition of a surfactant.

The absorbent foam of the present invention has been found to be able tobe prepared by a relatively simple, safe, and cost-effective process. Inone embodiment, the process generally comprises forming a solution of asoluble polymer in a solvent, freezing the solution at a relatively slowcooling rate to a temperature below the freezing point of the solvent,removing the solvent from the frozen solution, and optionally treatingthe polymer to form a water-swellable, water-insoluble polymericabsorbent foam.

In another embodiment, the process comprises forming a solution ofmonomers in a solvent, polymerizing the monomers to form a solution gelof a crosslinked polymer in the solvent, freezing the solution gel at arelatively slow cooling rate to a temperature below the freezing pointof the solvent, and removing the solvent from the frozen solution gel.Optionally, the solution gel of the crosslinked polymer could besubjected to additional swelling, by using additional solvent, beforefreezing the solution gel.

The absorbent foam of the present invention is also believed to becapable of being formed by a process generally comprising forming asolution of a soluble polymer in a solvent, adding a blowing agent tothe solution, initiating the blowing agent, removing the solvent fromthe solution, and optionally treating the polymer to form awater-swellable, water-insoluble polymeric absorbent foam.

As used herein, the term "solvent" is intended to represent a substance,particularly in a liquid form, that is capable of dissolving the polymerused herein to form a substantially uniformly dispersed mixture at themolecular level. In one embodiment of the present invention, the solventused to prepare the absorbent foam needs to be capable of first freezingand then be capable of undergoing sublimation, wherein the solventpasses directly from its frozen state to a vapor state. As such, thesolvent used to prepare the absorbent foam should have a freezing pointat which the solvent changes from a liquid to a solid. The freezingpoint of water and other solvents is generally well known in the art.However, as will be recognized by one skilled in the art, the freezingpoint of a particular solvent may be affected by such factors as theparticular solvent, polymer and the crosslinking agents being used aswell as the relative concentrations of the respective components in thesolution.

The soluble polymer or the monomers are typically dissolved in a solventcomprising at least about 30 weight percent water, beneficially about 50weight percent water, suitably about 75 weight percent water, and moresuitably 100 weight percent water. When a co-solvent is employed withthe water, other suitable solvents include methanol, ethanol, acetone,isopropyl alcohol, ethylene glycol, glycerol, and other solvents knownin the art. However, when a water-soluble polymer is used, the use orpresence of such other, non-aqueous solvents may impede the formation ofa homogeneous mixture such that the polymer does not effectivelydissolve into the solvent to form a solution.

In general, a solution of the polymer, the solvent and, optionally, acrosslinking agent and/or other optional components is prepared, whereinthe polymer may be added to the solution as a polymer or formed as apolymer in the solution from monomers. In the present invention, it hasbeen discovered that controlling the concentration of the polymer in thesolution is important to achieving an absorbent foam that exhibits thedesired properties. In general, if the concentration of the polymer inthe solution is too high, the resultant absorbent foam prepared has beenfound to not exhibit the desired properties, particularly softness, dueto the formation of relatively thick cell walls. Without intending to bebound hereby, it is hypothesized that the use of too great of aconcentration of the polymer in the solution results in a relativelysmall volume of space occupied by solvent molecules as compared to theoverall solution volume. In general, if the concentration of the polymerin the solution is too low, the resultant absorbent foam prepared hasbeen found to not exhibit the desired properties, particularly absorbentproperties and liquid distribution capability, due to the formation ofcell walls that are too thin and cells that are too large. Withoutintending to be bound hereby, it is hypothesized that the use of toosmall of a concentration of the polymer in the solution results in toomuch volume of space occupied by the solvent molecules. It has generallybeen found that the higher the concentration of the polymer in thesolution, the resulting absorbent foam exhibits a smaller average cellsize and thicker average cell walls as compared to an absorbent foamprepared from a solution with a lower concentration of the polymer inthe solution.

Thus, it is generally desired that the solution comprises from about 0.1to about 30 weight percent, beneficially from about 0.5 to about 20weight percent, and suitably from about 1 to about 10 weight percent,based on total solution weight, of the polymer. The solution generallycomprises from about 99.99 to about 70 weight percent, beneficially fromabout 99.5 to about 80 weight percent, and suitably from about 99 toabout 90 weight percent of the solvent.

In one embodiment of the present invention, the dissolution of a solublepolymer into a solvent is believed to result in entanglement ofindividual segments of the polymer chains with each other. Suchentanglement results in the polymer chains interpenetrating one anotherin the mixture, so that a random, coil-entangled molecular configurationoccurs which is believed to effectively provide crosslinking points andwhich assists allowing for additional crosslinking of the polymer uponfurther treatment as, for example, with heat-treatment. To allow foreffective entanglement of individual segments of the polymer with eachother, the solution is suitably allowed to form a stable, homogeneoussolution at equilibrium prior to additional treatment steps to ensureeffective dissolution of the polymer into the solvent. It will beappreciated that a relatively minor amount of a non-soluble portion ofthe polymer may exist that will typically not dissolve into the solvent.For example, the retained crystalline areas of a crystalline-crosslinkedpolymer will typically not dissolve in water while the non-crystallineareas typically will.

Generally, the order of mixing the polymer or monomers, the solvent and,optionally, any crosslinking agents is not critical. As such, either thepolymer, the monomers, or the crosslinking agent may be added to thesolvent and then the remaining component subsequently added, or allcomponents may be added together at the same time. However, it may bebeneficial, when using certain crosslinking agents, to first add thepolymer or monomer and solvent and then to add the crosslinking agent tothe solution.

The solution of the polymer or monomers, solvent and, optionally, acrosslinking agent can generally be formed at any temperature at whichthe polymer or monomers is soluble in the solvent. Generally, suchtemperatures will be within the range of from about 10° C. to about 100°C.

The solution may be acidic (a pH of less than 7), neutral (a pH of 7),or basic (a pH greater than 7). If desired, the solution can beacidified by the addition of an aqueous solution of an inorganic acid,such as hydrochloric acid or nitric acid, or an aqueous solution of anorganic acid, such as acetic acid. Similarly, if it is desired toprovide the solution with a basic pH, a base such as an aqueous solutionof sodium hydroxide, potassium hydroxide, or ammonia can be added to thesolution.

The solution will generally have a pH within the range of from about 2to about 12, beneficially from about 4 to about 9, more beneficiallyfrom about 4 to about 7.5, and suitably from about 6 to about 7.5. Theresulting absorbent foam will generally have the same pH as thesolution.

When the absorbent foam of the present invention is intended for use inpersonal care products, such as diapers, training pants, and femininecare products, it is typically desired that the absorbent foam have agenerally neutral character. For this reason, it is generally beneficialthat the solution be formed with a generally neutral pH. If the solutionis formed with an acidic or basic pH, the recovered absorbent foam maybe acidic or basic (respectively) but may be neutralized. A recoveredabsorbent foam which is acidic may be neutralized, for example, bycontacting it with a gaseous base such as ammonia. A recovered absorbentfoam which is basic may be neutralized, for example, by contacting itwith an acidic gas such as carbon dioxide.

After forming the solution comprising the polymer or monomers, solventand, optionally, a crosslinking agent, the solution is beneficiallyagitated, stirred, or otherwise blended to effectively uniformly mix thecomponents such that an essentially homogeneous solution is formed.

If monomers are being used, the monomers are suitably then treated toform the desired polymer in the solution.

The solution is then cooled to a temperature that is below the freezingpoint of the solvent such that the solvent freezes and becomes a solidphase in the solution. Since the polymer and, optionally, a crosslinkingagent are essentially homogeneously dispersed in the solution, it isgenerally desired that the polymer and, optionally, the crosslinkingagent form an essentially continuous matrix within the frozen solutionwhen the solvent freezes and becomes a solid phase. As such, theessentially continuous matrix of the polymer and, optionally, thecrosslinking agent will become substantially encased by the frozensolvent, forming an essentially uniform bicontinuous structure. As usedherein, the term "encase" and related terms are intended to mean thatthe frozen solvent phase substantially encloses or surrounds theessentially continuous matrix of the polymer and, optionally, thecrosslinking agent.

As will be recognized by one skilled in the art, the temperature towhich the solution is cooled in order to freeze the solvent willtypically depend on such factors as the solvent, the polymer and thecrosslinking agent being used as well as the relative concentrations ofthe respective components in the solution. In general, it has been foundthat if the temperature to which to solution is eventually cooled is tooclose to the freezing point of the solvent, the frozen polymer solutionmay not exhibit sufficient strength and may deform under furtherprocessing steps such as under vacuum treatment to remove the frozensolvent. In addition, the freezing point of the solvent may be depresseddue to the effect of the dissolved polymer and/or crosslinking agent. Assuch, if the solution is merely cooled to the freezing point of the puresolvent, then some of the solvent present in the solution may not be inthe frozen state at such a temperature. In general, it has also beenfound that if the temperature to which to solution is eventually cooledis too far below the freezing point of the solvent, molecules of thesolvent in the solution may tend to form a non-uniform crystalline phasethroughout the solution which has been found to often cause theformation of cracks in the polymer matrix and thus in the absorbent foamthat is being prepared. Such cracks tend to reduce the mechanicalproperties of the absorbent foam, such as tensile strength and softnessor flexibility. In addition, the use of very low temperatures tends toslow down the rate of subliming the frozen solvent.

In one embodiment where the solvent used to prepare the absorbent foamis essentially all water or an aqueous solution comprising mostly waterbut also another solvent, it is generally desired that the temperatureto which to solution is eventually cooled to be between about -50° C.and about 0C., beneficially between about -50° C. and about -5° C., morebeneficially between about -40° C. and about -10° C., and suitablybetween about -30° C. and about -10° C.

It has also been found that the rate at which the solution is cooledfrom a temperature above the freezing point of the solvent to atemperature below the freezing point of the solvent is important toachieving an absorbent foam that exhibits the desired propertiesdescribed herein. In a qualitative manner, the cooling rate used shouldbe not be so fast that visible cracks or visible non-uniformities beginto form in the freezing solution. As such, there is generally a criticalcooling rate that will exist for a particular solution in order toachieve a desired absorbent foam of the present invention. Using acooling rate that is faster than such a critical cooling rate willgenerally result in an undesirable absorbent foam that exhibits arelatively non-uniform pore structure and cracked polymer matrix. Incontrast, using a cooling rate that is slower than such a criticalcooling rate will generally result in a desirable absorbent foam thathas a relatively uniform pore structure and the absence of anysignificant cracks or deformities in the polymer matrix.

As with the freezing point of a solvent, the critical cooling rate to beused for a particular solution will typically depend on such factors asthe solvent, the polymer and the crosslinking agent being used as wellas the relative concentrations of the respective components in thesolution. In one embodiment of the present invention, wherein water isthe solvent or an aqueous solution comprising mostly water but alsoanother solvent and, more particularly, wherein the polymer is used in aconcentration of between about 0.5 to about 2 weight percent wherein theweight percent is based on the total weight of the solvent, the criticalcooling rate has been found to be a decrease in temperature betweenabout 0.4° C. to about 0.5° C. per minute. In such an embodiment, it istherefore desired that the cooling rate used to freeze the solvent beless than about 0.4° C. per minute, beneficially less than about 0.3° C.per minute, and suitably less than about 0.1° C. per minute.

As will be recognized by one skilled in the art, besides the approach ofusing a cooling rate slower than a critical cooling rate to achieve anessentially uniform cell structure in the absorbent foam, other methodscan also be applied. Such other methods include, but are not limited to,the inclusion of tiny air bubbles or the use of a nucleating agent.Without intending to be bound hereby, it is hypothesized that the use ofa nucleating agent will increase the number of nuclei to ensure anessentially uniform crystallization of solvent molecules during thecooling process. Use of a nucleating agent generally increases thecritical cooling rate.

After the solution has been cooled such that the solvent freezes andbecomes a solid phase in the solution and the solution has beneficiallyreached a relatively stable temperature, the frozen solvent is thensubstantially removed from the solution. In the present invention, theuse of a suitable vacuum to sublime the frozen solvent has been found togenerally result in a desired absorbent foam. As will be appreciated byone skilled in the art, the vacuum to be used for a particular frozensolution will typically depend on such factors as the solvent, thepolymer and the crosslinking agent being used, the relativeconcentrations of the respective components in the solution, and thetemperature of the frozen solution. Desirable vacuum conditions arebeneficially less than about 500 millitorrs, more beneficially less thanabout 300 millitorrs, suitably less than about 200 millitorrs, and moresuitably less than about 100 millitorrs. In general, the higher thevacuum, the faster the rate of sublimation of the frozen solvent.

As used herein, the sublimation, by use of a vacuum, of the frozensolvent from the frozen solution is meant to represent thatsubstantially all of the solvent is removed from the frozen solutionprior to, if needed, any additional treatment steps. It will beappreciated, however, that even after removal of substantially all ofthe solvent, a small amount of solvent may remain entrapped within thestructure of the remaining polymeric matrix. The amount of solventremaining entrapped within the structure of the polymeric matrix willtypically depend on the method and conditions under which the frozensolvent is sublimed. Generally, less than about 20 weight percent,beneficially less than about 15 weight percent, and suitably less thanabout 10 weight percent, of the original amount of solvent in thesolution will remain entrapped within the remaining polymeric matrix ofthe absorbent foam.

After the frozen solvent has been substantially sublimed from the frozensolution, the polymer and, optionally, any crosslinking agent willremain, with the polymer generally forming a polymeric matrix comprisinggenerally interconnected cells to achieve a foam structure. Thepolymeric matrix forms the walls of the cells with the open cells havingbeen created by the sublimation of the frozen solvent. As discussedhereinbefore, it is generally desired that the resultant foam structureexhibit a desired average pore size and a desired average thickness ofthe cell walls.

The recovered foam structure may already exhibit the desired absorbentand physical properties such that the recovered foam structure is anabsorbent foam of the present invention and does not require any furthertreatment steps. As will be appreciated by one skilled in the art, thiswill generally depend on the particular polymer and, if used, theparticular crosslinking agent used in the preparation of the foam.Methods of preparation wherein the recovered foam structure may alreadyexhibit the desired absorbent and physical properties include whereinmonomers were polymerized in the solution to form a crosslinked and thusinsoluble polymer gel solution; a crosslinking agent that is capable ofreacting with the polymer at a relatively low temperature, such as atabout room temperature or less, is used; and the polymer used, such aspolyvinyl alcohol or chitosan, is capable of forming a highly orderedstructure during the freezing and solidification process.

If the recovered foam structure does not yet exhibit the desiredabsorbent and physical properties, it may be necessary to treat therecovered polymeric foam structure with an additional process step. Forexample, if the crosslinking agent used is a latent crosslinking agent,such a crosslinking agent may not yet have reacted with the polymerbecause the proper crosslinking condition has not yet been provided tothe polymer and crosslinking agent mixture. As such, an effectivecrosslinking condition may still need to be provided in order tocrosslink the polymer to achieve a water-insoluble, water-swellablepolymer. Suitable post treatment conditions include using heattreatment, exposure to ultraviolet light, exposure to microwaves,exposure to an electron beam, steam or high humidity treatment, highpressure treatment, or treatment with an organic solvent.

In general, if heat-treatment is necessary, any combination oftemperature and time which is effective in achieving a desired degree ofcrosslinking, without undesirable damage to the polymer, so that thepolymer and the absorbent foam exhibit the desired properties describedherein, is suitable for use in the present invention. As a general rule,when a crosslinking agent is used, the polymer will be heat-treated at atemperature between about 50° C. to about 250° C., beneficially fromabout 80° C. to about 250° C., more beneficially from about 100° C. toabout 200° C., and suitably from about 100° C. to about 160° C. Thehigher the temperature employed, the shorter the period of timegenerally necessary to achieve the desired degree of crosslinking. Ithas been found that if very high temperatures are used with an effectivelength of time, such as a temperature between about 100° C. and about250° C. for a length of time between about 50 seconds and about 500minutes, effective Free Swell and Absorbency Under Load values may beachieved for certain polymers, such as carboxyalkyl polysaccharidewithout the use of a crosslinking agent.

Generally, the heat-treating process will extend over a time periodwithin the range of from about 1 minute to about 600 minutes,beneficially from about 2 minutes to about 200 minutes, and suitablyfrom about 5 minutes to about 100 minutes.

If used, a heat-treating process, or any other acceptable post-recoverytreatment process, generally causes the polymer to crosslink oradditionally crosslink and become generally water swellable and waterinsoluble. Without intending to be bound hereby, it is believed that thepost-recovery treatment processes cause the polymer to undergo a degreeof crosslinking, not related to the presence of a crosslinking agent,through the formation of crosslinks between either the functional groupsfrom the polymer and the external crosslinking agent or between thefunctional groups on the polymer when it contains more than one type offunctional groups. One example of a self-crosslinkable polymer iscarboxymethyl cellulose, which contains both carboxylic acid groups andhydroxyl groups and is able to form ester linkages. Thisself-crosslinking may be in addition to any crosslinking caused by thepresence of a crosslinking agent. Further, when the crosslinking agentis a diamine or polyamine, it is believed that crosslinking occursthrough amidation of any carboxyl groups on the polymer through theformation of an ammonia salt. Esterification, through aself-crosslinking process, is believed to occur primarily under a weaklyacidic, neutral, or slightly basic condition. Esterification, through aself-crosslinking process, is not believed to proceed to a significantdegree under relatively basic conditions. Crosslinking due to thecrosslinking agent may occur under both acidic and basic conditions.Thus, the presence of the crosslinking agent allows for crosslinking tooccur over a broad pH range.

There is generally an optimum degree or amount of crosslinking of aparticular polymer that optimizes the absorbency properties of theparticular crosslinked polymer. If too little crosslinking occurs, thepolymer may possess relatively low absorbency properties, such asAbsorbency Under Load values, due to a lack of gel strength. If too muchcrosslinking occurs, the polymer may similarly possess relatively lowabsorbency properties, such as Free Swell values, due to the inabilityof the polymer to absorb liquid.

Those skilled in the art will recognize that the presence of crosslinksformed by esterification or amidation, ionic bonding, or other types oflinkages can be detected through various analytical techniques. Forexample, infrared spectroscopy and nuclear magnetic resonance can beused to verify the presence of ester and amide crosslinks.

The absorbent foams of the present invention are suited for use indisposable products including disposable absorbent products such asdiapers, adult incontinent products, and bed pads; in catamenial devicessuch as sanitary napkins, and tampons; and other absorbent products suchas wipes, bibs, wound dressings, and surgical capes or drapes.Accordingly, in another aspect, the present invention relates to adisposable absorbent product comprising the absorbent foams of thepresent invention.

In one embodiment of the present invention, a disposable absorbentproduct is provided, which disposable absorbent product comprises aliquid-permeable topsheet, a backsheet attached to the liquid-permeabletopsheet, and an absorbent structure positioned between theliquid-permeable topsheet and the backsheet, wherein the absorbentstructure comprises an absorbent foam of the present invention.

Absorbent products and structures according to all aspects of thepresent invention are generally subjected, during use, to multipleinsults of a body liquid. Accordingly, the absorbent products andstructures are desirably capable of absorbing multiple insults of bodyliquids in quantities to which the absorbent products and structureswill be exposed during use. The insults are generally separated from oneanother by a period of time.

Test Methods

Free Swell

The Free Swell Capacity (FS) is a test which measures the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in 1 hour under a negligibleapplied load or restraining force, such as of about 0.01 pound persquare inch.

Referring to FIG. 1, the apparatus and method for determining the FreeSwell and the Absorbency Under Load will be described. Shown is aperspective view of the apparatus in position during a test. Shown is alaboratory jack 1 having an adjustable knob 2 for raising and loweringthe platform 3. A laboratory stand 4 supports a spring 5 connected to amodified thickness meter probe 6, which passes through the housing 7 ofthe meter, which is rigidly supported by the laboratory stand. A plasticsample cup 8, which contains the absorbent foam material sample to betested, has a liquid-permeable bottom and rests within a Petri dish 9which contains the saline solution to be absorbed. For the determinationof Absorbency Under Load values only, a weight 10 rests on top of aspacer disc (not visible) resting on top of the absorbent foam materialsample (not visible).

The sample cup consists of a plastic cylinder having a 1 inch insidediameter and an outside diameter of 1.25 inches. The bottom of thesample cup is formed by adhering a 100 mesh metal screen having 150micron openings to the end of the cylinder by heating the screen abovethe melting point of the plastic and pressing the plastic cylinderagainst the hot screen to melt the plastic and bond the screen to theplastic cylinder.

The modified thickness meter used to measure the expansion of the samplewhile absorbing the saline solution is a Mitutoyo Digimatic Indicator,IDC Series 543, Model 543-180, having a range of 0-0.5 inch and anaccuracy of 0.00005 inch (Mitutoyo Corporation, 31-19, Shiba 5-chome,Minato-ku, Tokyo 108, Japan). As supplied from Mitutoyo Corporation, thethickness meter contains a spring attached to the probe within the meterhousing. This spring is removed to provide a free-falling probe whichhas a downward force of about 27 grams. In addition, the cap over thetop of the probe, located on the top of the meter housing, is alsoremoved to enable attachment of the probe to the suspension spring 5(available from McMaster-Carr Supply Co., Chicago, Ill., Item No.9640K41), which serves to counter or reduce the downward force of theprobe to about 1 gram+0.5 gram. A wire hook can be glued to the top ofthe probe for attachment to the suspension spring. The bottom tip of theprobe is also provided with an extension needle (Mitutoyo Corporation,Part No. 131279) to enable the probe to be inserted into the sample cup.

To carry out the test, an absorbent foam material sample was cut intocircular discs with a diameter of about one inch. A total of about 0.160gram of the absorbent foam material sample, typically about 3 to 4circular disc layers, is placed into the sample cup. The sample is thencovered with a plastic spacer disc, weighing 4.4 grams and having adiameter of about 0.995 inch, which serves to protect the sample frombeing disturbed during the test and also to uniformly apply a load onthe entire sample. The sample cup, with material sample and spacer disc,is then weighed to obtain its dry weight. The sample cup is placed inthe Petri dish on the platform and the laboratory jack raised up untilthe top side of the plastic spacer disc contacts the tip of the probe.The meter is zeroed. A sufficient amount of saline solution is added tothe Petri dish (50-100 milliliters) to begin the test. The distance theplastic spacer disc is raised by the expanding sample as it absorbs thesaline solution is measured by the probe. This distance, multiplied bythe cross-sectional area inside the sample cup, is a measure of theexpansion volume of the sample due to absorption. Factoring in thedensity of the saline solution and the weight of the sample, the amountof saline solution absorbed is readily calculated. The weight of salinesolution absorbed after about 1 hour is the Free Swell value expressedas grams saline solution absorbed per gram of absorbent foam sample. Ifdesired, the readings of the modified thickness meter can becontinuously inputted to a computer (Mitutoyo Digimatic MiniprocessorDP-2 DX) to make the calculations and provide Free Swell readings. As across-check, the Free Swell can also be determined by determining theweight difference between the sample cup before and after the test, theweight difference being the amount of solution absorbed by the sample.

Absorbency Under Load

The Absorbency Under Load (AUL) is a test which measures the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in 1 hour under an appliedload or restraining force of about 0.3 pound per square inch. Theprocedure for measuring the Absorbency Under Load value of an absorbentcomposition is essentially identical to the procedure for measuring theFree Swell values, except that a 100 gram weight is placed on top of theplastic spacer disc, thereby applying a load of about 0.3 pound persquare inch onto the absorbent foam as it absorbs the saline solution.

Softness

The Softness value of a material is determined by a test which ismodeled after the ASTM D4032-82 Circular Bend Procedure. This modifiedtest is used for the purposes of the present invention and is,hereinafter, simply referred to as the "Circular Bend Procedure". TheCircular Bend Procedure is a simultaneous multi-directional deformationof a material in which one face of a material becomes concave and theother face becomes convex. The Circular Bend Procedure gives a forcevalue which relates to the stiffness of the material, simultaneouslyaveraging stiffness in all directions, and is herein as being inverselyrelated to the softness of the material.

The apparatus necessary for the Circular Bend Procedure is a modifiedCircular Bend Stiffness Tester, having the following parts: Asmooth-polished steel plate platform which is 102.0 millimeters (length)by 102.0 millimeters (width) by 6.35 millimeters (depth) having a 18.75millimeter diameter orifice. The lap edge of the orifice should be at a45 degree angle to a depth of 4.75 millimeters. A plunger having thefollowing dimensions is used: overall length of 72.2 millimeters, adiameter of 6.25 millimeters, a ball nose having a radius of 2.97millimeters and a needle-point extending 0.88 millimeters from the ballnose with a 0.33 millimeter base diameter and a point having a radius ofless than 0.5 millimeters. The plunger is mounted concentrically withthe orifice having equal clearance on all sides. The needle-point isused merely to prevent lateral movement of a sample during testing. Thebottom of the plunger should be set well above the top of the orificeplate. From this position, the downward stroke of the ball nose is tothe exact bottom of the plate orifice.

An inverted compression load cell having a load range of from about 0.0to about 2000.0 grams was used as a force measurement gauge. Thecompression tester used was an Instron Model No. 1122 invertedcompression load cell, available from Instron Engineering Corporation ofCanton, Mass.

After calibrating the load cell, the gage length for displacement of theplunger was set to 25.4 mm. To carry out the test, an absorbent foamsample was cut into a 38.1×38.1 mm square specimen using a die cutter.The sample was placed onto the test platform and the plunger was lowereddown on the specimen for a 25.4 mm gage length at a crosshead speed of500 mm/min. During the movement of the plunger, the absorbent foamsample is deflected downward into the 18.75 mm hole by the plunger andthe force exerted by the compression tester to deflect the foam sampleduring the 25.4 mm gage length displacement of the plunger is measuredby the load cell and recorded. The force measured by the load celldivided by the basis weight of the specimen is reported in units ofgrams force/grams per square meter of specimen (g/gsm). This value isused as the Softness value to obtain a quantitative measure of thesoftness of the specimen. The higher the Softness value (in g/gsm), themore stiff and, thus, the less soft, the specimen.

Cell Pore Size and Cell Wall Thickness Measurements

A foam sample was cut by a sharp razor. The cut foams were attached tometal stubs using copper tape and imaged in an environmental scanningelectron microscope using 12 kV beam voltage. The instrument used was anenvironmental scanning electron microscope, model E-2020 fromElectroscan Corporation of Wilmington, Mass. The sample chamber pressurewas about 1.2 Torr. The environmental backscatter electron detector wasused to collect images, having the advantage of being able to discernany variations in composition. Magnification varied depending on thescale of the subject sample, with a 150 magnification used for a generalsurvey of the sample and a 2500 magnification used to measure cell wallthickness and cell size. Cell wall thickness and cell size measurementswere taken directly on the environmental scanning electron microscope.It was not possible to apply automated image analysis routines to thesecomplex structures for cell wall thickness measurement. Manualmeasurement is required. The cell wall thickness and cell size of eachsample are averaged from at least measurements.

EXAMPLES

For use in the following examples, the following polymer materials wereobtained.

Polymer 1: A carboxymethylcellulose having a weight average molecularweight greater than 1,000,000 and a degree of substitution ofcarboxymethyl groups on the anhydroglucose unit of the cellulosicmaterial of about 0.7 was obtained from Aqualon of Wilmington, Del., asubsidiary of Hercules Inc., under the designation B313carboxymethylcellulose. Carboxymethylcellulose is an anionic polymer.

Polymer 2: A sodium polyacrylate polymer having a weight averagemolecular weight of about 4,000,000 and degree of neutralization ofabout 70 percent was obtained from Polysciences of Warrington, Pa.,under the catalog number of 06501. Sodium polyacrylate polymer is ananionic polymer.

Polymer 3: A sodium polyacrylate polymer having a weight averagemolecular weight of about 240,000 and degree of neutralization of about70 percent was obtained from Polysciences of Warrington, Pa., under thecatalog number of 18613. Sodium polyacrylate polymer is an anionicpolymer.

Polymer 4: A sodium polyacrylate polymer having a weight averagemolecular weight of about 60,000 and degree of neutralization of about70 percent was obtained from Polysciences of Warrington, Pa., under thecatalog number of 18611. Sodium polyacrylate polymer is an anionicpolymer.

Polymer 5: A chitosan acetate having a weight average molecular weightof about 11,000,000 and degree of acetylation of about 80 percent wasobtained from Vanson Company of Seattle, Wash., under the designationVNS-608 chitosan. Chitosan acetate is a cationic polymer.

Polymer 6: A polyethyleneoxide having a weight average molecular weightof about 4,000,000 was obtained from Union Carbide Corporation ofDanbury, Conn., under the designation WSR-301 polyethyleneoxide.Polyethyleneoxide is a nonionic polymer.

Example 1

Weight amounts of the various polymer samples were dissolved in separatebatches of about 2000 grams of distilled water at a temperature of about23° C. For the carboxymethylcellulose (Polymer 1) and polyethylene oxide(Polymer 6) solutions, about 0.2 gram of citric acid was also added tothe solutions as a crosslinking agent. For the sodium polyacrylatesolutions (Polymers 2-4) solutions, about 0.75 gram of an aqueoussolution comprising about 40 weight percent of ammonium zirconiumcarbonate was also added to the solutions as a crosslinking agent. Thevarious solutions were blended for about 2 to 3 hours to ensure thoroughmixing of the components. About 500 grams of each prepared solution wasplaced into separate stainless steel pans, wherein the pans haddimensions of 10 inches (width) by 20 inches (length) by 1 inch (depth).The pans, containing the respective solutions, were then placed in afreeze dryer, available from The VirTis, Inc., of Gardiner, N.Y., underthe designation VirTis Genesis model 25EL freeze dryer. The varioussolutions in the pans were then cooled down to about -15° C. at variouscooling rates in order to freeze the water in the solutions. The varioussolutions in the pans were maintained at about -15° C. for about an hourto ensure substantially complete freezing of the water. The frozensolutions were left in the freeze dryer and then subjected to a vacuumof about 105 millitorrs, provided by a vacuum pump which had a condenserset to a temperature of about -60° C. to about -70° C., for about 15hours. The resultant foam structures were then treated at varioustemperatures for various periods of time in order to assist in thecrosslinking of the polymers. The final foam structures were thenevaluated for Free Swell, Absorbency Under Load, and Softness values.The various process conditions and results of the evaluations for thevarious samples are summarized in Table 1. The foam sample preparedusing Polymer 4 (Sample 7) was water soluble and therefore did notexhibit any measurable Free Swell and Absorbency Under Load values.

A comparative foam sample (Sample 10) was also prepared, as follows.About 250 grams of aqueous acrylic acid solution containing 50 percentby weight of acrylic acid was neutralized using 1 N sodium hydroxidesolution to form sodium acrylate solution with 75 percent degree ofneutralization. The neutralization was carried out slowly using an icebath taking care to maintain the solution temperature around 5° C. toavoid any polymerization. About 200 ml of this solution was transferredto a 2 liter reaction vessel fitted with a heating jacket and a highshear mixer (Ultra-Turrax T25 mixer from Janke & Kunkel GmbH of Staufen,Germany). To the solution in the reaction vessel was added 0.5 grams ofN,N'-methylenebisacrylamide, about 1.3 grams of2,2'-azobis-(2-amidopropane) hydrochloride from Monomer-Polymer & DajacLaboratories, Inc. of Feasterville, Pa., and about 20 grams ofpolyethylene glycol of 600 weight average molecular weight from UnionCarbide Company maintaining the mixture at 22° C. About 3.5 grams ofsorbitan monolaurate and about 6.5 grams of ethoxylated sorbitanmonolaurate were mixed in about 60 grams of1,1,2-trichlorotrifluoroethane and this mixture was then added to thesolution in the reaction vessel. The high shear mixer was started andthe mixture was mixed at a speed of about 8000 rpm for about 10 minutesafter which the mixer was removed from the reaction vessel and thetemperature increased to about 60° C. and maintained for about 1 hour toform the foam, followed by increasing and maintaining the temperature atabout 80° C. for about 30 minutes and finally increasing and maintainingthe temperature at about 120° C. for about 30 minutes. The reactor wasthen cooled to about 22° C. A mixture consisting of about 5 grams ofglycerol and about 25 grams of isopropyl alcohol was added to the foamin the reactor and the temperature was increased to about 180° C. andmaintained for about 1 hour. The reactor was then cooled to ambienttemperature, the foam removed from the reactor and placed in a chamberat about 80 percent relative humidity for about 6 hours to obtain thefinal foam material. This foam sample was then evaluated for Free Swell,Absorbency Under Load, and Softness values, with the results of suchevaluations also summarized in Table 1.

Example 2

Multiple, substantially similar foam samples were prepared as follows.About 10 grams of Polymer 1 (carboxymethylcellulose) was dissolved inabout 2000 grams of distilled water at a temperature of about 23° C.About 0.2 gram of citric acid was also added to the solution as acrosslinking agent. The solution was blended for about 2 to 3 hours toensure thorough mixing of the components. About 500 grams of thesolution was placed into a stainless steel pan, wherein the pan haddimensions of 10 inches (width) by 20 inches (length) by 1 inch (depth).The pan, containing the solution, was then placed in freeze dryer,available from The VirTis, Inc. of Gardiner, N.Y., under the designationVirTis Genesis model 25EL freeze

                                      TABLE 1                                     __________________________________________________________________________               Polymer      Heat                                                             Concentration                                                                              Treatment      Absorbency                                                                          Softness                               Polymer                                                                            (Weight                                                                              Cooling                                                                             Conditions                                                                             Free Swell                                                                          Under Load                                                                          Value                            Sample No.                                                                          Type Percent)                                                                             Rate  (Temperature/Time)                                                                     Value (g/g)                                                                         Value (g/g)                                                                         (g/gsm)                          __________________________________________________________________________    Sample 1                                                                            Polymer 1                                                                          0.5    0.03° C./min                                                                 130° C./10 min                                                                  26.1  18.3  2.90                             Sample 2                                                                            Polymer 1                                                                          0.5    0.2° C./min                                                                  130° C./10 min                                                                  21.8  --    6.53                             Sample 3                                                                            Polymer 1                                                                          0.5    0.4° C./min                                                                  130° C./10 min                                                                  22.1  --    20.58                            *Sample 4                                                                           Polymer 1                                                                          4.0    0.4° C./min                                                                  130° C./10 min                                                                  18.7  --    45.93                            Sample 5                                                                            Polymer 2                                                                          0.5    0.03° C./min                                                                 200° C./40 min                                                                  35.0  19.5  1.36                             Sample 6                                                                            Polymer 3                                                                          0.5    0.03° C./min                                                                 200° C./5 hours                                                                 42.8  2.5   1.09                             *Sample 7                                                                           Polymer 4                                                                          0.5    0.03° C./min                                                                 200° C./72 hours                                                                0     0     1.54                             Sample 8                                                                            Polymer 5                                                                          0.5    0.03° C./min                                                                 100° C./10 min                                                                  22.5  14.3  3.26                             Sample 9                                                                            Polymer 6                                                                          0.5    0.03° C./min                                                                 60° C./10 hours                                                                 15.3  4.1   0.46                             *Sample 10 --     --    --       15    10.5  >100                             __________________________________________________________________________     *Not an example of the present invention.                                

dryer. The solution in the pan was then cooled down to about -15° C. ata cooling rate of about 0.04° C./minute in order to freeze the water inthe solution. The solution in the pan were maintained at about -15° C.for about an hour to ensure substantially complete freezing of thewater. The frozen solutions were left in the freeze dryer and thensubjected to a vacuum of about 105 millitorrs, provided by a vacuum pumpwhich had a condenser set to a temperature of about -60° C. to about-70° C., for about 15 hours.

The resultant foam structures were then treated at various temperaturesfor various periods of time in order to assist in the crosslinking ofthe polymers. The final foam structures were then evaluated for FreeSwell, Absorbency Under Load, and Softness values. The various processconditions and results of the evaluations for the various samples aresummarized in Table 2.

                  TABLE 2                                                         ______________________________________                                                Heat                                                                          Treatment                                                                     Conditions            Absorbency                                                                            Softness                                        (Temperature/                                                                            Free Swell Under Load                                                                            Value                                   Sample No.                                                                            Time)      Value (g/g)                                                                              Value (g/g)                                                                           (g/gsm)                                 ______________________________________                                        *Sample 11                                                                            None       0          0       20.58                                   Sample 12                                                                             150° C./5 min                                                                     27.9       --      --                                      Sample 13                                                                             150° C./10 min                                                                    25.1       14.3    --                                      Sample 14                                                                             150° C./20 min                                                                    15.7       11.2    --                                      Sample 15                                                                             150° C./30 min                                                                    16.6       11.3    20.58                                   *Sample 16                                                                            None       0          0       --                                      Sample 17                                                                             130° C./5 min                                                                     60.1       29.9    20.58                                   Sample 18                                                                             130° C./10 min                                                                    26.1       18.3    --                                      Sample 19                                                                             130° C./15 min                                                                    21.8       16.2    --                                      Sample 20                                                                             130° C./20 min                                                                    18.2       14.6    --                                      Sample 21                                                                             130° C./25 min                                                                    17.4       13.9    20.58                                   ______________________________________                                         *Not an example of the present invention.                                

Those skilled in the art will recognize that the present invention iscapable of many modifications and variations without departing from thescope thereof. Accordingly, the detailed description and examples setforth above are meant to be illustrative only and are not intended tolimit, in any manner, the scope of the invention as set forth in theappended claims.

What is claimed is:
 1. An absorbent foam comprising a water-swellable,water-insoluble polymer wherein the water-swellable, water-insolublepolymer is present in the absorbent foam in a weight amount betweenabout 50 weight percent to 100 weight percent, based on the total weightof the absorbent foam, and wherein the absorbent foam exhibits a FreeSwell value of at least about 10 grams of liquid per gram of absorbentfoam and a Softness value that is less than about 30 grams of force pergram per square meter of the absorbent foam.
 2. The absorbent foam ofclaim 1 wherein the polymer is selected from the group consisting ofpolyacrylamides, polyvinyl alcohols, ethylene maleic anhydridecopolymer, polyvinylethers, polyacrylic acids, polyvinylpyrrolidones,polyvinylmorpholines, polyamines, polyethyleneimines, polyquaternaryammoniums, carboxymethyl celluloses, carboxymethyl starchs,hydroxypropyl celluloses, algins, alginates, carrageenans, acrylicgrafted starchs, acrylic grafted celluloses, chitin, chitosan,polyaspartic acid, polyglutamic acid, polyasparagins, polyglutamines,polylysines, polyarginines, and the salts, copolymers, and mixtures ofany of the foregoing polymers.
 3. The absorbent foam of claim 2 whereinthe polymer is selected from the group consisting of polyacrylic acids,carboxymethyl celluloses, chitin, chitosan, and the salts, copolymers,and mixtures of any of the foregoing polymers.
 4. The absorbent foam ofclaim 3 wherein the water-swellable, water-insoluble polymer is selectedfrom the group consisting of polyacrylic acids and its salts.
 5. Theabsorbent foam of claim 1 wherein the water-swellable, water-insolublepolymer is present in the absorbent foam in a weight amount betweenabout 60 weight percent to 100 weight percent.
 6. The absorbent foam ofclaim 1 wherein the absorbent foam further comprises a crosslinkingagent.
 7. The absorbent foam of claim 6 wherein the crosslinking agentis selected from the group consisting of an organic compound having atleast two functional groups or functionalities capable of reacting withthe polymer and a metal ion with two or more positive charges.
 8. Theabsorbent foam of claim 6 wherein the crosslinking agent is present inthe absorbent foam in a weight amount between about 0.01 weight percentto about 20 weight percent, based on the total weight of the absorbentfoam.
 9. The absorbent foam of claim 1 wherein the absorbent foamexhibits a Free Swell value of at least about 15 grams of liquid pergram of absorbent foam.
 10. The absorbent foam of claim 1 wherein theabsorbent foam exhibits a Softness value that is less than about 25grams of force per gram per square meter of the absorbent foam.
 11. Theabsorbent foam of claim 1 wherein the absorbent foam exhibits anAbsorbency Under Load value of at least about 10 grams of liquid pergram of absorbent foam.
 12. The absorbent foam of claim 1 wherein theabsorbent foam comprises cells and wherein the average cell size of thecells is between about 10 microns to about 100 microns.
 13. Theabsorbent foam of claim 1 wherein the absorbent foam comprises cellscomprising walls having a thickness and wherein the average wallthickness of the cells is between about 0.1 micron to about 30 microns.14. The absorbent foam of claim 1 wherein the water-swellable,water-insoluble polymer is selected from the group consisting ofpolyacrylic acids, carboxymethyl celluloses, chitin, chitosan, and thesalts, copolymers, and mixtures of any of the foregoing polymers,wherein the absorbent foam comprises cells comprising walls having athickness wherein the average cell size of the cells is between about 10microns to about 100 microns, and wherein the average wall thickness ofthe cells is between about 0.1 micron to about 30 microns.
 15. Anabsorbent foam comprising a water-swellable, water-insoluble polymer,wherein the water-swellable, water-insoluble polymer is present in theabsorbent foam in a weight amount between about 50 weight percent toabout 100 weight percent, wherein the absorbent foam comprises cellscomprising walls having a thickness wherein the average cell size of thecells is between about 10 microns to about 100 microns, and wherein theaverage wall thickness of the cells is between about 0.1 micron to about30 microns, further wherein the absorbent foam has a softness value thatis less than about 30 grams of force per gram per square meter of theabsorbent foam.
 16. The absorbent foam of claim 15 wherein the polymeris selected from the group consisting of polyacrylamides, polyvinylalcohols, ethylene maleic anhydride copolymer, polyvinylethers,polyacrylic acids, polyvinylpyrrolidones, polyvinylmorpholines,polyamines, polyethyleneimines, polyquaternary ammoniums, carboxymethylcelluloses, carboxymethyl starchs, hydroxypropyl celluloses, algins,alginates, carrageenans, acrylic grafted starchs, acrylic graftedcelluloses, chitin, chitosan, polyaspartic acid, polyglutamic acid,polyasparagins, polyglutamines, polylysines, polyarginines, and thesalts, copolymers, and mixtures of any of the foregoing polymers. 17.The absorbent foam of claim 16 wherein the water-swellable,water-insoluble polymer is selected from the group consisting ofpolyacrylic acids, carboxymethyl celluloses, chitin, chitosan, and thesalts, copolymers, and mixtures of any of the foregoing polymers. 18.The absorbent foam of claim 17 wherein the water-swellable,water-insoluble polymer is selected from the group consisting ofpolyacrylic acids and its salts.
 19. The absorbent foam of claim 15wherein the water-swellable, water-insoluble polymer is present in theabsorbent foam in a weight amount between about 60 weight percent to 100weight percent.
 20. The absorbent foam of claim 15 wherein the absorbentfoam further comprises a crosslinking agent.
 21. The absorbent foam ofclaim 20 wherein crosslinking agent is selected from the groupconsisting of an organic compound having at least two functional groupsor functionalities capable of reacting with the polymer and a metal ionwith two or more positive charges.
 22. The absorbent foam of claim 21wherein the crosslinking agent is present in the absorbent foam in aweight amount between about 0.01 weight percent to about 20 weightpercent, based on the total weight of the absorbent foam.
 23. Theabsorbent foam of claim 15 wherein the absorbent foam exhibits a FreeSwell value of at least about 10 grams of liquid per gram of absorbentfoam.
 24. A disposable absorbent product comprising a liquid-permeabletopsheet, a backsheet attached to the topsheet, and an absorbent corepositioned between the liquid-permeable topsheet and the backsheet,wherein the absorbent core comprises an absorbent foam, wherein theabsorbent foam comprises a water-swellable, water-insoluble polymerwherein the water-swellable, water-insoluble polymer is present in theabsorbent foam in a weight amount between about 50 weight percent to 100weight percent, based on the total weight of the absorbent foam, andwherein the absorbent foam exhibits a Free Swell value of at least about10 grams of liquid per gram of absorbent foam and a Softness value thatis less than about 30 grams of force per gram per square meter of theabsorbent foam.
 25. A disposable absorbent product comprising aliquid-permeable topsheet, a backsheet attached to the topsheet, and anabsorbent core positioned between the liquid-permeable topsheet and thebacksheet, wherein the absorbent core comprises an absorbent foam,wherein the absorbent foam comprises a water-swellable, water-insolublepolymer wherein the water-swellable, water-insoluble polymer is presentin the absorbent foam in a weight amount between about 50 weight percentto 100 weight percent, wherein the absorbent foam comprises cellscomprising walls having a thickness wherein the average cell size of thecells is between about 10 microns to about 100 microns, and wherein theaverage wall thickness of the cells is between about 0.1 micron to about30 microns, further wherein the absorbent foam has a softness value thatis less than about 30 grams of force per gram per square meter of theabsorbent foam.